518 PHYSIOLOGY OF GROWTH. 



and blade, thus left unhampered, at first causes the leaves to 

 assume the position indicated in Fig. 174. After a short time, 

 however, in the pulvinus of the blade, and possibly also in the 

 leaf-stalk, there appears a curvature (not a torsion) whereby 

 the lamina is directed obliquely frontwards to the light (see 

 Fig. 176). We see, therefore, that the leaves, even in complete 

 absence of gravitational influence, can come to a fixed light 

 position. And in doing so, as is specially to be emphasised, no 

 torsions are effected. Under other circumstances, however, as we 

 have seen, torsions play an important role in connection with the 

 movements of orientation of leaves. 



1 See Stahl, Berichte d. Deutschen botan. Gesellscli., Bd. 2, H. 8. 



2 See Wiesner, Denksch. d. Akad. d. Wiss. in Wien, Bd. 43. 



8 See H. de Vries, Arbeiten d. bntan. Inst. in Wiirzburg, Bd. 1, p. 223. 



4 See Detmer, Botan. Zeitung, 1882, No. 46. 



5 Literature : Frank, Die natiirliche tvagerechte Stellung der Pflanzentheile, 

 Leipzig, 1870 ; C. Darwin. The Power of Movement in Plants ; F. Darwin, J. 

 Linn. Soc., Botany, Vol. 18 (De% 1830) ; Schmidt, Dissertation, Berlin, 1883 ; 

 Noll, Arbeiten d. botan. Inst. in Wiirzburg, Bd. 3 ; Krabbe, Pringsheim's Jahrb., 

 Bd. 20 ; Vochting, Botan. Zeitung, 1888 ; Noll, Ueber heterogene Induction, 

 Leipzig, 1892 ; Schwendener and Krabbe, Abliandl. d. Konigl. preuss. Akad. d. 

 Wiss., Berlin, 1892 ; Noll, Die Orientirungsbewegungen dorsiventraler Orcjane, 

 Munchen, 1892. 



195. General Considerations Respecting the Rigidity of Plant 



Structures. 



Plants, like animals, need special contrivances to enable them 

 to withstand certain external influences and maintain their form. 

 In many cases turgidity and tissue tensions play an important 

 part as a means of strengthening the organism, but Schwendener's 

 researches l have farther proved that plants are characterised by 

 a more or less connected mechanical system (stereome), which 

 performs functions quite similar in many respects to those of the 

 bony skeleton of higher animals. 



As the ultimate organs (stereides) of the mechanical system, we 

 have especially to take into consideration the sclerenchyma fibres 

 in the ground tissue, the true bast fibres, the collenchyma, and the 

 libriform fibres. The sclerenchyma cells and the true bast fibres 

 are elongated elements, whose membranes, frequently lignified, 

 are much thickened (see Fig. 177). Like these in many respects 



